1. Field of the Invention
The present disclosure relates to a cutting tool for boring, and more particularly, to a cutting tool for boring (also called a boring bit) which is used for turning the inner circumferential surface of a hole having a circular cross-section, or the inner circumferential surface of a cylindrical pipe, particularly for machining a deep small-diameter hole.
2. Description of the Related Art
Cutting tools for boring are used to machine the inner circumferential surface of a hole of a workpiece (an object to be cut), such that the cutting edge (the front end of a cutting edge for cutting) of a cutting insert (for example, a throw-away type cutting insert) fixed to the front end of a bar-shaped holder overhangs from a side of the outer circumferential surface of the holder. In the cutting tool for boring (hereinafter, also referred to as a cutting tool), when the diameter of the inner circumferential surface of a hole of a workpiece (hereinafter, also referred to as an inner circumferential surface) is small, for example, below φ12 mm, the protrusion amount of the cutting edge from the outer circumferential surface of the holder is about 1.5 mm at most, though it depends on the outer diameter of the holder (normally about 10 mm). Therefore, the gap (space) between the inner circumferential surface of the hole and the outer circumferential surface of the holder becomes too small, such that it is difficult to remove the chips produced in the cutting process from the inlet of the hole of the workpiece at the rear end (base) of the holder (also called a shank) to the outside (rear end side of the holder). Further, the deeper the hole in the cutting conditions, the more serious the problem becomes. As the performance of the chip control (removing) degrades, the chips are easily stuck between the inner circumferential surface of the machined hole and the cutting edge or the holder. Accordingly, the machined surface (inner circumferential surface) is damaged, and the machined surface is roughened. Further, in some cases, machining is inevitably stopped in order to remove the chips. This problem tends to occur in particular in an external oil supply type apparatus in which cutting oil is supplied from the outside, rather than in an internal oil supply type apparatus in which cutting oil is supplied from the front end of the holder.
As a countermeasure considered for controlling the chips, the portion of the holder including the front end in particular which is inserted into the inner circumferential surface of the hole of a workpiece is thinned, in addition to controlling the shape, state, or removing direction of the chips. However, simply thinning the holder causes chatter vibration due to a decrease in stiffness. Therefore, in order to more efficiently remove the chips without causing chatter vibration, a technology has been proposed in which various shapes of chip pockets (concave portions, hereinafter also referred to as pockets) are formed at the head (the portion close to the front end) of the holder where the cutting insert is fixed. This technology intends to effectively remove the chips produced in the inner surface machining to the outside by forming the pockets in the proper places and using them as channels or guides.
For example, a cutting tool for boring (boring bit) has been proposed that has a pocket (front end-sided chip pocket) formed concavely with respect to the outer circumferential surface of a holder above the cutting surface of a cutting insert that is fixed to the front end of a holder; and a chip pocket inclined upward toward the rear end of the head of the holder (shank) and connected to the pocket (space above the cutting surface). According to this cutting tool, a first sub-pocket (cutting surface-sided chip pocket) is formed to extend forward and backward on the outer circumferential surface of the holder where the cutting edge overhangs, in the upper side of the cutting surface of the cutting insert. A second sub-pocket is formed to extend forward and backward on the outer circumferential surface of the holder at the opposite side to the side where the cutting edge overhangs, in the upper side of the cutting surface of the cutting insert. Also, a rib extending forward and backward is formed between the first sub-pocket and the second sub-pocket in the holder, and the front surface (front end surface) of the rib is connected to the inclined pocket. See, for example, Japanese Patent Application Laid-Open No. 2005-279855-A.
In cutting (machining an inner surface) by the cutting tool for boring described above, the following effects (1) and (2) can be achieved on discharging the chips.
(1) Chips produced in machining the inner circumferential surface of a hole and comminuted into in small sizes are received in the first sub-pocket formed on the outer circumferential surface of the holder at the side of the cutting edge from the chip pocket that has received the chips first, and then removed to the outside of the machined hole through the first sub-pocket.
(2) The chips not comminuted are received in the second sub-pocket formed on the outer circumferential surface of the holder at the circumferential opposite side with respect to the cutting edge from the chip pocket, and then removed outside through the second sub-pocket. Therefore, less chips are stuck or come between the machined hole and the holder, such that a very accurate hole can be machined.
3. Problems to be Solved by the Invention
When a sample of a cutting tool having the above configuration (minimum machinable inner diameter: φ10 mm) is used for a cutting test, the following results (1) and (2) are confirmed. The cutting test is performed in two cases where chips are comminuted (sheared) in small sizes (workpiece: stainless steel SUS303) and where chips are relatively continuous (workpiece: stainless steel SUS304), by changing the shape of a breaker of a cutting insert. Here, the test conditions include the diameter (inner diameter) of the hole: 10 mm, hole depth: 20 mm, cut depth: 0.25 mm, cutting feed: 0.05 mm/rev, cutting speed: 80 m/min, and external cutting oil supply.
(1) In the cutting in which chips are comminuted into small sizes, the chips mainly flow only to the first sub-pocket. However, since the chip pocket is small as a concave space, it is difficult to remove the chips to the outside from the chip pocket, such that the chips tend to stick and remain on the inner circumferential surface. Therefore, a quality of the roughness on the machined surface degrades in a finish cutting.
(2) While the continuous chips (relatively long chips) flow to the second sub-pocket as well, the continuous chips tend to flow into the first sub-pocket instead. It appears that this is because chips remain even though removed to the second sub-pocket. Further, since the chip pocket of the first sub-pocket is small in this case as well as a concave space, the chips tend to remain between the first sub-pocket (concaved surface) and the inner circumferential surface of the hole. Therefore, the chips are easily wound and stuck to the cutting edge or the holder, such that a quality of the roughness on the machined inner circumferential surface degrades.
That is, when a relatively small-diameter and deep hole is machined in an external oil supply type apparatus by a cutting tool for boring of the related art, the chips tend to flow into the first sub-pocket and also remain in the hole, regardless of their shape. This phenomenon is seen by observing the removed state of the chips in the cutting process, or by observing the inner circumferential surface of the hole of the workpiece in the process of taking out the cutting tool after cutting is stopped, or in the process of taking out the cutting tool after a deep hole is machined.